1. Field of the Invention
The invention relates generally to communications technology, and more particularly to techniques for characterizing a multiplicity of signals occurring on a given communications medium.
2. Description of Related Art
Presently-existing communications systems have been developed to convey a plurality of signals over a common communications medium. One example of such a communications system is conventional analog cable television, which transmits a plurality of analog audiovisual signals on a single coaxial cable. These systems essentially represent a type of frequency-division multiplexing, wherein a first analog communications channel is assigned a first frequency band, a second analog communications channel is assigned a second frequency band, and so on. More recently, mixed-signal communications systems have evolved which are equipped to convey digital, as well as analog, signals. Such digital signals may be employed, for example, to represent audiovisual information, data, and/or control signals. Note that the same modulation scheme need not be applied to all signals on a given communications medium. For example, some signals may be modulated using quadrature amplitude modulation (QAM), and other signals may be modulated using single sideband (SSB), vestigial sideband (VSB), pulse amplitude modulation (PAM), and/or FM.
FIG. 1 is an illustrative prior-art mixed-signal communications system. A composite signal includes a plurality of signals, such as first signal 31, second signal 32, third signal 33, fourth signal 34, fifth signal 35, sixth signal 36, seventh signal 37, and eighth signal 38. Fourth signal 34 may represent a conventional analog vestigial sideband (VSB) modulated television signal, second signal 32 may represent a QAM (quadrature amplitude modulated) channel of relatively wide bandwidth, and first signal 31 may represent a QAM channel of narrower bandwidth relative to the second signal 32.
Head end 10 is used to receive, process, and retransmit the composite signal. Head end 10 is coupled to the input of a modem 12 via a first section of coaxial cable. The modem 12 modulates the signals transmitted by head end 10 onto the first section of coaxial cable using conventional modem communications protocols. The output of modem 12 is fed to an electrical-to-optical interface 14 via a second section of coaxial cable. Electrical-to-optical interface 14 converts the electrical signals generated by modem 12 into optical pulses suitable for transmission over fiber-optic cable. If coaxial cable were used to convey signals over relatively great distances, signal attenuation could be a problem. The use of fiber-optic cable allows signal transmission over relatively large distances.
An optical-to-electrical interface 16 converts the optical pulses on the fiber optic cable back into electrical signals suitable for transmission over coaxial cable, in preparation for signal delivery to customer premises equipment 20. A section of coaxial cable links optical-to-electrical interface 16 to modem 18, and another section of coaxial cable links modem 18 to customer premises equipment 20. Modem 18 demodulates the signal on coaxial cable 40, so that a specified portion of the composite signal transmitted by head end 10 is delivered to customer premises equipment 20 over coaxial cable 41.
Modem 18 uses a tuning mechanism to demodulate the signal that will be sent to customer premises equipment 20. In practice, this tuning mechanism includes a mixer/oscillator stage that mixes signals within a specified bandwidth on coaxial cable 40 down to a lower frequency range. Since this mixer/oscillator stage is not synchronized with respect to head end 10, the modem 18 may provide a demodulated signal that has a frequency offset with respect to the signals originally transmitted by the head end 10.
Upon initial startup, the tuning mechanism may, due to unpredictable frequency offset adjustments, provide a composite signal mix to the customer premises equipment that contains only a portion of a QAM signal at the lower edge of the RF-demodulated frequency spectrum, as is shown in FIG. 1 just to the left of third signal 33 at customer premises equipment 20. This is an undesired situation, as the customer premises equipment 20 will not be able to properly process a QAM signal by receiving only the band edge of that signal. What is needed is automatic readjustment of the tuner mixer/oscillator stage, based upon a knowledge of the spectral arrangement of the composite signal, to ensure that all desired QAM signals are fully-contained within the available RF demodulation bandwidth.